The present invention relates to a dual polarization transmission system and, more particularly, to a dual polarization transmission system in which channels having the same center frequency are arranged on two orthogonal polarized waves in a digital radio communication system.
In order to increase a channel capacity in a digital radio communication system having a large number of channels, an interleave frequency arrangement in which channels are alternately arranged on two orthogonal polarized waves, e.g., a vertically polarized wave and a horizontally polarized wave has been widely used.
FIG. 1 shows an arrangement of a conventional system for realizing an interleave frequency arrangement, and FIG. 2 shows the interleave frequency arrangement.
In FIG. 1, a transmitter side is constituted by 16 QAM modulators 111, 112, 131, and 132 for outputting 16 QAM modulated signals (IF signals) having a bandwidth B1, transmitters 211, 212, 231, and 232 for converting the 16 QAM modulated signals into signals of radio frequencies having center frequencies f1, f3, f2, and f4 and outputting the obtained signals, and a transmitting antenna 101. Outputs from the transmitters 211 and 212 are connected to a vertical polarization side of the transmitting antenna 101 and transmitted as vertically polarized channels 11 and 12 having the center frequencies f1 and f3 and the bandwidth B1. Outputs from the transmitters 231 and 232 are connected to a horizontal polarization side of the transmitting antenna 101 and transmitted as horizontally polarized channels 31 and 32 having the center frequencies f2 and f4 and the bandwidth B1. As a result, the interleave frequency arrangement shown in FIG. 2 is realized.
A receiving side is constituted by receivers 311, 312, 331, and 332 and 16 QAM demodulators 411, 412, 431, and 432. A vertical polarization side of a receiving antenna 102 is connected to the receivers 311 and 312. The receivers 311 and 312 separate the channels 11 and 12, respectively, and frequency-convert the channels into signals having an IF band. The IF signals are demodulated by the 16 QAM demodulators 411 and 412. A horizontal polarization side of the receiving antenna 102 is connected to the receivers 331 and 332. The receivers 331 and 332 separate the channels 31 and 32, respectively, and frequency-convert the channels into signals having an IF band The IF signals are demodulated by the 16 QAM demodulators 431 and 432.
In the interleave frequency arrangement shown in FIG. 2, the vertically polarized channels 11 and 12 and the horizontally polarized channels 31 and 32 are alternately arranged on a frequency axis. Since polarized wave discrimination is possible between two adjacent channels, channel interference can be sufficiently reduced even if the bands of the two channels more or less overlap each other. Therefore, the bandwidth of the channels is set such that the bands of the channels slightly overlap each other.
FIG. 3 is a block diagram of an arrangement of a dual polarization transmission system which has been used to further improve a frequency band utilizing efficiency in recent years and in which channels having the same center frequency are arranged on two orthogonal polarized waves. FIG. 4 shows a frequency arrangement in the system of FIG. 3. In FIGS. 3 and 4, the same reference numerals as in FIGS. 1 and 2 denote the same parts.
In the conventional example in FIG. 3, vertically polarized channels 51 and 52 and horizontally polarized channels 61 and 62 are added to the interleave frequency arrangement shown in FIG. 2 such that center frequencies of the channels 51, 52, 61, and 62 coincide with the center frequencies f2, f4, f1, and f3 of the channels 31, 32, 11, and 12, respectively.
In FIG. 3, in order to add the vertically polarized channels 51 and 52 having the bandwidth B1 and the center frequencies f2 and f4, respectively, shown in FIG. 4, to the system shown in FIG. 1, 16 QAM modulators 151 and 152 and transmitters 251 and 252 are additionally connected to the vertically polarization side of the transmitting antenna 101, respectively. In addition, in order to receive and demodulate the channels 51 and 52, receivers 351 and 352 and 16 QAM demodulators 451 and 452 are connected to the vertically polarization side of the receiving antenna 102. In order to add the horizontally polarized channels 61 and 62 having the bandwidth B1 and the center frequencies f1 and f3, 16 QAM modulators 161 and 162 and transmitters 261 and 262, and receivers 362 and 363 and 16 QAM demodulators 462 and 463 are connected to the horizontally polarization sides of the transmitting and receiving antennas 101 and 102, respectively.
Since the bandwidth of all the channels is the same, portions (hatched portions in FIG. 4) in which the bands overlap each other are produced between adjacent channels having the same polarized wave, thereby increasing channel interference. Therefore, the bandwidth of each channel must be narrowed.
In order to narrow the bandwidth of each channel, a roll-off coefficient is reduced to strictly limit the bandwidth, or a code transmission rate is reduced.
Data to be transmitted by each channel is composite data obtained by multiplexing main data which is information to be transmitted, subdata such as a digital service channel, and overhead bits for monitoring and controlling a radio section such as a frame sync bit and a parity check bit. By increasing or reducing the number of the subdata or the overhead bits, the code transmission rate can be increased or reduced to some extent even at the same information transmission rate.
As described above, according to the conventional dual polarization transmission system, the bandwidth of each channel must narrowed. However, if the bandwidth is strictly limited for this purpose, a transmission quality is degraded due to nonlinearity, and no system gain can be obtained. In addition, it is difficult to realize a filter system, and degradation in hardware tends to occur. Meanwhile, when the code transmission rate is reduced to narrow the bandwidth, a transmission capacity of the subdata is reduced to limit applications of the system, or a capacity of the overhead bits is reduced to disable high-grade monitoring or control of lines. Furthermore, when the bandwidth of existing channels in the interleave frequency arrangement is narrowed in order to additionally provide new channels, modification of equipment costs a great deal.